Digital image sensing based upon solid state technology is well known, the two most common types of image sensors currently being charge coupled devices (CCD's) and complementary metal oxide semiconductor (CMOS) image sensors. Digital image sensors are incorporated within a wide variety of devices throughout the consumer, industrial and defense sectors among others. Typical consumer products that incorporate digital image sensors include for example digital still cameras, mobile devices such as mobile telephones, webcams, laptops and desktop computers, and video recorders.
An image sensor is a device comprising one or more radiation sensitive elements having an electrical property that changes when radiation is incident upon them, together with circuitry for converting the changed electrical property into a signal. As an example, an image sensor may comprise a photodetector that generates a charge when radiation is incident upon it. The photodetector may be designed to be sensitive to electromagnetic radiation in the range of (human) visible wavelengths, or other neighboring wavelength ranges, such as infra red or ultra violet for example. Circuitry is provided that collects and carries the charge from the radiation sensitive element for conversion to a value representing the intensity of incident radiation.
Typically, more than one radiation sensitive element will be provided in an array. The term pixel is used as a shorthand for picture element. In the context of a digital image sensor, a pixel refers to that portion of the image sensor that contributes one value representative of the radiation intensity at that point on the array. These pixel values are combined to reproduce a scene that is to be imaged by the sensor. A plurality of pixel values can be referred to collectively as image data. Pixels are usually formed on and/or within a semiconductor substrate. In fact, the radiation sensitive element comprises only a part of the pixel, and only part of the pixel's surface area (the proportion of the pixel area that the radiation sensitive element takes up is known as the fill factor). Other parts of the pixel are taken up by metallization such as transistor gates and so on. Other image sensor components, such as readout electronics, analogue to digital conversion circuitry and so on may be provided at least partially as part of each pixel, depending on the pixel architecture.
An image sensor is typically provided on or as part of an integrated circuit. The image sensor may also be provided with other components such as infra-red filters, color filter arrays and so on. It is also known to provide an image sensor with so called microlenses which are formed from optically transparent material over each individual pixel in the array, to focus light onto the light sensitive portions of the pixel, rather than onto portions which are not sensitive to light.
An image sensor system, or camera, that comprises a digital image sensor will comprise an image sensor chip (which may include microlenses formed or provided thereupon), and an optical element such as a lens or other waveguide for manipulating incident light onto the image plane of the image sensor. This optical element is known as the objective optical element, objective lens, or simply “objective”.
In all application areas, there is a desire to minimize the size of the package that houses a camera. This need is particularly pressing in the field of mobile devices, where demands for increasing number of device functions and increasing complexity of functions leads to a strong desire for space saving.
In a traditional camera arrangement the physical size of the area where the objective optical element produces an image determines the focal length and thus the required height of the assembly that houses the objective optical element, called the “optical assembly” or as a shorthand simply the “optics”. This height is referred to in the art as z-height—the x, y and z axes forming an orthogonal axis set and the x and y directions being in the plane of the image sensor. However, reducing the z-height in order to reduce the thickness of the camera also means reducing the sensor area and thus the sampled image resolution.
In order to achieve a reduced z-height it is known to employ a compound imaging geometry. Instead of a single objective optical element being provided which directs incident radiation on to the pixel array, two or more objectives are provided, each of which directs radiation onto a different subset of the pixel array. Each objective optical element together with the relevant subset of pixels provides a “channel” of a so called multi-channel image sensor. As an example, if the sensor area is divided into four sub areas of equal size then each sub sensor size is reduced by two. Correspondingly the focal length and thus the optical height are also reduced by a factor of two. Various compound imaging geometries have been disclosed to date, as seen for example U.S. Pat. Nos. 6,137,535; 6,141,048; and 6,765,617 and United States Patent Application Publication 2005/0128335, the disclosures of which are hereby incorporated by reference.
These solutions however suffer from parallax between channels and from color aliasing. Improvements in these areas are much desired to make a reduced z-height optical system using compound imaging geometry viable.